Choice of method

The choice of good practice method depends on national circumstances. The decision tree in Figure 3.2 describes good practice in adapting the methods to national circumstances. Emissions can be estimated from:

• continuous emissions monitoring (CEM) where emissions are directly measured at all times;

• periodic emissions monitoring that is undertaken over a period(s) that is reflective of the usual pattern of operation of the plant to derive an emission factor that is multiplied by output (100 percent HNO3) to derive emissions;

• irregular sampling to derive an emission factor that is multiplied by output (100 percent HNO3) to derive emissions;

• by multiplying a default emission factor by output (100 percent HNO3).

Methods are classified according to the extent of plant-level data that are available. Both Tier 2 and Tier 3 are based on plant-level activity data.

TIER 1 METHOD

Emissions are estimated as follows:

EN2O = N2O emissions, kg

Where:

EN2O = N2O emissions, kg

EF = N2O emission factor (default), kg N2O/tonne nitric acid produced

NAP = nitric acid production, tonnes

When applying the Tier 1 method it is good practice to assume that there is no abatement of N2O emissions and to use the highest default emission factor based on technology type shown in Table 3.3.

TIER 2 METHOD

There are a large number of nitric acid plants (estimates range from 255 to 600 plants according to Choe et al, 1993; Bockman and Granli, 1994) with substantial variations in the N2O generation factors among plant types. Consequently, default factors may be needed more often for nitric acid N2O emissions estimates. Where default values are used to estimate emissions from nitric acid production, it is good practice to categorise plants according to type and to use an appropriate N2O generation factor.

The Tier 2 method uses plant-level production data disaggregated by technology type and default emission factors classified by technology type. Emissions are calculated as follows:

Equation 3.6

N2O emissions from nitric acid production - Tier 2

Where:

EN2O = emissions of N2O, kg

EFi = N2O emission factor for technology type i, kg N2O/tonne nitric acid produced

NAPi = nitric acid production from technology type i, tonnes

DFj = destruction factor for abatement technology type j, fraction

ASUFj = abatement system utilisation factor for abatement technology type j, fraction

Note that the default emission factors shown in Table 3.3 include the impact on emissions of abatement technology where relevant. To use these factors, inventory compilers should verify that the abatement technology is installed at individual plants and operated throughout the year.

The basic equation for estimating N2O emissions includes additional terms that recognise the potential future use of N2O abatement technologies. The N2O destruction factor has to be multiplied by an abatement system utilisation factor in order to account for any down-time of the emission abatement equipment (i.e., time the equipment is not operating).

TIER 3 METHOD - DIRECT MEASUREMENT

While the tier 2 approach applies technology specific emission factors reflecting the national technology mix. Tier 3 is based on real measurement data (e.g., CEMS). Plant-level production data disaggregated by technology type and plant level emission factors obtained from direct measurement of emissions. These may be derived from irregular sampling of emissions of N2O or periodic emissions monitoring of N2O undertaken over a period(s) that reflects the usual pattern of operation of the plant. Emissions can be derived using Equation 3.6.

Alternatively, the Tier 3 method uses the results of continuous emissions monitoring (CEM), although it is noted that most plants are unlikely to employ CEM due to the resource costs. Where CEM is employed, emissions can be estimated based on the sum of measured N2O emissions derived from the concentration of N2O in monitored emissions for each recorded monitoring interval.

Figure 3.2 Decision tree for estimation of N2O emissions from nitric acid production

Figure 3.2 Decision tree for estimation of N2O emissions from nitric acid production

Note:

1. See Volume 1 Chapter 4, Methodological Choice and Identification of Key Categories (noting Section 4.1.2 on limited resources), for discussion of key categories and use of decision trees.

Note:

1. See Volume 1 Chapter 4, Methodological Choice and Identification of Key Categories (noting Section 4.1.2 on limited resources), for discussion of key categories and use of decision trees.

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